Atrial fibrillation (AF) is a growing epidemic with ~16 million Americans affected by 2050. Despite recent advances in catheter-based therapy, antiarrhythmic drugs (AADs) are still commonly used to treat patients with symptomatic AF. However, response in an individual patient is highly variable and can be associated with significant toxicities. The limited success of AADs in treating AF is due in part to heterogeneity of the underlying substrate and our inability to predict individual responses to therapy. Thus, a major knowledge gap is predicting which patients with AF are likely to respond to antiarrhythmic therapy. Emerging evidence supports the overarching hypothesis to be tested here that variability in response to AADs is modulated by a single nucleotide polymorphism (SNP) associated with AF. Although genetic approaches to AF have revealed that susceptibility to and response to therapy is modulated by the underlying substrate, the translation of these discoveries to the bedside care of patients has been limited in part because of poor understanding of the underlying mechanisms by which risk alleles cause AF, challenges associated with determining the therapeutic efficacy and lack of prospective genotype-guided studies. Aim 1 will test the hypothesis that a common chromosome (chr) 4q25 AF SNP modulates response to AADs in patients with symptomatic AF using burden as a measure of therapeutic efficacy. The scientific premise for this proposal is based on our published study which showed that a chr4q25 SNP not only predicted successful symptom control of AF but that patients who carried the risk allele responded better to Na+-channel than K+-channel blocker AADs; a recent study that confirmed our clinical observation; and preliminary data generated in our pilot and feasibility study. We propose a randomized cross-over study whereby patients will be given flecainide/sotalol and therapeutic efficacy will be assessed by implanting insertable cardiac monitors. While we showed that AF patients who carry the chr4q25 AF risk allele are more likely to respond to flecainide than those who carry the wild-type (WT) allele, the underlying mechanism for this differential response to AADs is poorly understood. Aim 2 will elucidate the underlying cellular mechanisms by which a chr4q25 risk SNP differentially modulates response to AADs in patients with AF using human atrial induced pluripotent stem cell derived cardiomyocytes (hiPSC-CMs). First, we will generate atrial iPSC-CMs from chr4q25 risk and WT allele carriers. Second, we will test the hypothesis that the electrophysiologic (EP) and structural maturity of atrial iPSC-CMs can be enhanced by precise microenvironmental engineering of in-vivo relevant cell-cell, cell-extracellular matrix, and cell-soluble factor interactions. Third, we will determine the EP phenotypes of mature atrial iPSC-CMs from chr4q25 AF risk and WT allele carriers and examine the effects of flecainide and sotalol ex-vivo. The proposed studies will not only improve the prediction of response to AADs for AF patients and pave the way for a genotype-guided approach but also facilitate the practice of personalized medicine.